The present invention relates to control components for a nuclear reactor and, more particularly, to a retainer to provide supplemental holddown force for undriven control components to preclude lifting of the component from the core by coolant flow.
Typically, a nuclear reactor for the generation of electrical power includes a core of fissionable material to heat a coolant flowing therethrough. The fissionable material is enclosed in elongated fuel rods assembled in a square array commonly called fuel assemblies. The fuel assemblies are held in an array by core grid plates at the top and bottom.
A control component assembly, i.e., a plurality of interconnected rods containing neutron absorbing material, disposed in guide tubes within a fuel assembly, is provided to control the nuclear fission reaction rate of the fuel assembly. Each rod in the control component assembly is bolted to and suspended from a spider, i.e., a structure centrally disposed above the fuel assembly having radially extending arms. Some types of control components, commonly referred to as control rods, contain a strong neutron absorber such as Ag-In-Cd or B.sub.4 C and are withdrawable from and insertable into the fuel assembly through the core grid plate, for starting and stopping, respectively, the nuclear fission chain reaction.
Another type of control component, commonly referred to as poison rods, contain weaker neutron absorbers such as B.sub.4 C in an Al.sub.2 O.sub.3 matrix. These rods usually are undriven and are not withdrawable. Rather, they are locked in position fully inserted in the fuel assemblies. Additional types of rods which are undriven and locked in position include: orifice plug rods, provided to preclude excess flow through guide tubes otherwise left open in fuel assemblies not requiring control component rods; and neutron source rods, orifice plug assemblies with at least one rod containing a source of neutrons provided for starting the reactor from total shutdown.
Locking control component assemblies into place in the fuel assembly is typically accomplished by a ball lock mechanism wherein the hub of the control rod assembly is insertable into a latch on the top of the fuel assembly and balls protruding from holes provided in the hub wall fit into a groove provided on the inside of the latch, thereby precluding the hub from being withdrawn from the latch. Typically in these types of locking mechanisms, the ball is not tight in the groove, thereby allowing some vertical movement of the control component assembly with respect to the fuel assembly.
It is necessary to allow considerable axial and lateral free play between the ball lock coupling mechanism and the latch in order to accommodate manufacturing dimensional tolerances, in service deformations and remote handling considerations. This free play, however, permits structural vibrations of the control component assembly to develop significant amplitude if unrestrained by some force. At low coolant flow velocities this is not a concern since the weight of the assembly is sufficient to hold it down on the top of the latch. In this condition, the locking balls do not even contact the latching groove due to the axial free play allowance. As the reactor flow is increased, however, the flow forces acting upward on the control component assembly increasingly counterbalance the downward force of gravity. Eventually, at a flow just below normal reactor full power conditions, the flow force exceeds the gravity force and the control component assembly lifts until its upward motion is limited when the locking balls contact the chamfered upper surface of the internal locking groove in the latch. In this nearly balanced condition the frictional forces available to resist relative motion between the control component assembly and the fuel assembly are very small. The turbulent high velocity coolant flow imparts significant driving energy into the hovering control component assembly and relatively large amplitude flow induced vibrations develop. Since the locking balls are now providing the limit to this motion the effect of this vibration is seen in a wearing away of the material on the upper chamfer of the locking groove.
On occasion, because of this flow induced vibration, latches of fuel assemblies in operating reactors have been worn to the extent that the locking mechanism has failed, resulting in ejecting of the control component assembly from the fuel assembly and damage to components of the reactor.
The present invention provides a supplemental holddown force to preclude lifting of the rods from the core.
A retainer engages both the top of the control component assembly and the bottom of the core grid plate. Additional holddown force is provided by the weight of the retainer and by a compressed spring disposed between the portion of the retainer engaging the control component assembly and the portion of the retainer engaging the core grid plate. One important advantage of this retainer is that it may be installed on currently operating reactors without alteration of fuel assemblies, control component assemblies or reactor internal structures. The retainer includes a housing to fit over the control component hub. A bottom plate affixed to the housing has slots to engage nuts on the control component assembly or alternatively, to engage the arm of the control component spider, thereby precluding rotation or horizontal translation of the retainer. Disposed within the housing and around the hub are a coil spring and a slidable ring. One end of each of the arms of the retainer is affixed to the ring and the other end engages the core grid plate. The spring acts to, in effect, apply expansion force between the control component assembly and the core grid plate via the structure of the retainer thus providing holddown force for the control component assembly and minimizing latch wear.
It is an object of the invention to provide a retainer to preclude ejection of control component assemblies from nuclear reactors.
A further object of the invention is a retainer that may be field loaded on irradiated components presently in use in operating nuclear reactors.
Another object of the invention is a retainer having the foregoing advantages and being capable of applying additional holddown force on the control component assembly.
A further object of the invention is a retainer having the foregoing advantages and being installable without modification of the nuclear reactor.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate the preferred embodiments of the present invention.